Microwave applicator with distributed feed to a resonator

ABSTRACT

A microwave applicator is disclosed which employs an elongated hollow resonator dimensioned to be cut off for the dominant waveguide mode at the frequency of the applicator. A hollow waveguide feed extends along and adjacent the resonator and includes a distributed coupling means providing a distributed wave energy feed path from the waveguide to the resonator. The distributed coupling means is arranged for exciting a transverse TEM mode in the resonator to produce standing wave microwave electric fields in the plane of a relatively thin elongated treatment zone within the resonator. The resonator includes a pair of elongated openings in registration with the treatment zone for passage of thin webs of material through the treatment zone. The distributed coupling means is preferably arranged to provide an increasing coefficient of coupling taken in the direction of power flow through the feed waveguide. Thus, the power is distributed to the resonator uniformly.

United States Patent [72] Inventor Norman H. Williams San Francisco, Calif. [21] Appl. No. 2,697 [22] Filed Jan. 14, 1970 [45] Patented Nov. 23, 1971 [73] Assignee Varian Associates Palo Alto, Calif.

[54] MICROWAVE APPLICATOR WITH DISTRIBUTED FEED TO A RESONATOR 12 Claims, 6 Drawing Figs.

[52] U.S.Cl 219/l0.55 [51] Int. Cl 1105p 9/06, HOSp 5/00 [50] Field of Search 219/1055; 3 3 3/2 1 [5 6] References Cited UNITED STATES PATENTS 3,439,143 4/1969 Cougoule 219/1055 3,507,050 4/1970 Smith etal ABSTRACT: A microwave applicator is disclosed which employs an elongated hollow resonator dimensioned to be cutoff for the dominant waveguide mode at the frequency of the applicator. A hollow waveguide feed extends along and adjacent the resonator and includes a distributed coupling means providing a distributed wave energy feed path from the waveguide to the resonator. The distributed coupling means is arranged for exciting a transverse TEM mode in the resonator to produce standing wave microwave electric fields in the plane of a relatively thin elongated treatment zone within the resonator, The resonator includes a pair of elongated openings in registration with the treatment zone for passage of thin webs of material through the treatment zone. The distributed coupling means is preferably arranged to provide an increasing coefficient ofcoupling taken in the direction ofpower flow through the feed waveguide. Thus, the power is distributed to the resonator uniformly.

PATENTEDNIJV 2a ISYI W I QQ FIG. 2

INVENTOR- NORMAN H. WILUAMS ATTORNEY DESCRIPTION OF THE PRIOR ART Heret ofore, microwave applicators have been constructed wherein a relatively large resonator structure was energized with microwave energy from a waveguide communicating with the fields of the resonator via an array of coupling slots or other coupling means. Such applicators have been constructed for treatment of relatively large masses of material,

- such as 50 to 100 billets, from which baseball bats are to be fabricated,.such billets being dried by the microwave energy supplied to the resonator. In such applicators, the large mass of the load greatly reduces the Q of the resonator such that the distributed feed of microwave energy to the resonator appears as an antenna for broadcasting microwave power from the feed points into the load. The cavity resonator has no resonant effect it merely serves an enclosure to prevent escape of stray radiation. Such an applicator is disclosed and claimed in copending US. application No. 821,176 filed May 2, 2969 and assigned to the same assignee as the present invention. While such an applicator is useful for treating large mass lossy loads, it is generally unsuited for treatment of small mass loads, such as paper webs and the like because such light mass loads do not sufficiently load the resonator and it is therefore reverberatory thereby destroying the uniformity of the electric field pattern generated within the enclosures due to standing waves set up within the enclosure. In the presence of the standing waves, in such a large enclosure, it is difficult to control the electric fields within the resonator in such a manner as to assure uniform treatment of the web of material being treated.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved microwave applicator of the type employing a resonator for treatment of relatively low mass work pieces.

One feature of the present invention is the provision, in a microwave applicator of the type employing a resonator portion fed with microwave power from a distributed coupling means disposed along the length of the resonator of dimensions for the resonator such that it is cut off, i.e. nonenergy propagating, for the dominant waveguide mode therein at the frequency of the applicator, and the provision of coupling means arranged for exciting the resonant transverse TEM mode in the resonator to the exclusion of exciting other possible modes, whereby a relatively high degree of mode control is attained such that uniform treatment can be obtained in a relatively thin elongated treatment zone extending longitudinally of the elongated resonator.

Another feature of the present invention is the same as the preceding feature wherein the resonator is of rectangular transverse section with a pair of ports disposed on opposite sidewalls of the resonator in alignment with the treatment zone for passage therethrough of material to be treated.

Another feature of the present invention is the same as any one or more of the preceding features wherein energy-absorbing means are disposed within the elongated resonator at opposite ends thereof to obtain a uniform electric field near the ends of the resonator.

Another feature of the present invention is the same as any one or more of the precedingfeatures wherein the waveguide and the elongated resonator are both of rectangular transverse section each having broad walls interconnected by narrow walls and wherein the width of the broad walls of the resonator is less than the width of the broad walls of the waveguide.

Another feature of the present invention is the same as any one or more of the preceding features wherein the coupling means includes means for providing a progressively increasing coupling coefficient to the resonator taken in the direction of power flow in the waveguide.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective line diagram partly broken away and depicting a microwave applicator of the present invention,

FIG. 2 is a reduced sectional view of the structure of FIG. 1 taken along the line 2-2 in the direction of the arrows, and having superimposed thereon a plot of the standing electric field intensity within the treatment zone versus distance along the length of the treatment zone.

FIG. 3 is a fragmentary plan view of an alternative to that portion of the structure of FIG. I depicted by a view taken along line 33 in the direction of the arrows.

FIG. 4 is a fragmentary perspective line diagram depicting an alternative embodiment of the present invention,

FIG. 5 is a reduced sectional view similar to that taken along line 5-5 of FIG. 1 and depicting an alternative embodiment of the present invention, and

FIG. 6 is a reduced sectional view similar to that taken along line 6-6 of FIG. 4 and depicting an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown a microwave applicator I incorporating features of the present invention. More particularly, the applicator 1 includes a hollow rectangular resonator portion 2 formed by a section of a length of rectangular waveguide 3 dimensioned to be cut off for the dominant longitudinal waveguide mode therein at the operating frequency of the applicator I. What this means is, that the broad walls 4 and 5 of the rectangular waveguide section 3 have a width w equal to or less than M2 where )t is a free space wavelength. In such a cutofi guide the guide wavelength becomes infinite such that the electric field vector is in the same direction for the full length of the resonator 2, thereby obtaining a very tight control over the electric field in the resonator 2.

Broad walls 4 and 5 ofthe resonator 2 each include an elongated access opening 6 and 7, respectively, through which a sheet of material 8 to be treated with microwave energy, such as paper, plastic films, or the like, may be passed to a relatively thin treatment zone within the resonator 2 which is in alignment with the apertures 6 and 7. The treatment zone is in a position of maximum electric field of the standing waves generated within the resonator 2. Moreover, the electric field vector of the standing wave is in the plane of the relatively thin treatment zone and therefore in the plane of the material to be treated, such as sheet 8.

The ends of the resonator 2 are preferably closed off by conductive walls 9 and II to prevent radiation of microwave energy out the ends of the resonator 2. In addition, a pair of microwave absorber structures 12 and 13, such as wedgeshaped resistive cards, are centrally disposed and axially directed from the ends walls 9 and 11 to resistively terminate the ends of the resonator such that a uniform electric field is provided for substantially the full length of the resonator within the treatment zone (see FIG. 2).

The rectangular resonator 2 includes a pair of narrow sidewalls l4 and 15 interconnecting the broad walls 4 and 5. Narrow wall 14 is common to an adjacent rectangular waveguide 16 which is parallel directed of the elongated resonator 2. A source of microwave energy 17, such as a klystron or a magnetron oscillator, is coupled to one end of the waveguide 16 via an E-plane bend transition waveguide section 18 for supplying microwave power to the waveguide 16.

The waveguide 16 includes a pair of mutually opposed broad walls 19 and 21 interconnected by a pair of narrow walls one of which is common wall 14. Common wall 14 includes an array of coupling holes 22 distributed along its length at intervals of preferably l guide wavelength in the standard waveguide 16 for coupling wave energy from the waveguide 16 into the resonator 2. The coupling holes are arranged for excitation of the transverse TEM mode within the elongated resonator 2, where such mode is defined taken in the direction normal to the common wall 14. Coupling holes 22 provide a relatively weak amount of coupling between the waveguide 16 and the cavity 2 and the coupling coefficient ther through from the waveguide 16 and the cavity 2 taken in the direction of power flow along the waveguide 16 from the source to a resistive load 23 connected to the opposite end of the waveguide 16. In this manner, as the power in the waveguide 16 decreased in the direction from the source 17 toward the load 23, an equal amount of power is coupled through each of the successive coupling holes 22 to the cavity 2 in order to provide a uniform transfer of power from the guide 16 to the resonator 2. over substantially the entire length of the resonator 2.

The advantage of the microwave applicator structure of FIG. 1 is that a relatively high degree of mode control is obtainable by dimensioning the resonator 2 in such a manner that the width of the resonator is cut off for the dominant waveguide mode therein but made resonant for the transverse TEM mode which is excited via the coupling holes 22 to the exclusion of other modes. Uniformity of the resultant standing wave electric field is shown in FIG. 2 by curve 24. When the resonator is made one-half a free space wavelength wide or less, the maximum electric field is positioned in the plane of the sheet material 8 being treated and is uniform over the entire length of the resonator 2 such that uniform treatment of the web 8 is obtained as the web passes through the treatment zone.

Referring now to FIG. 3, there is shown an alternative embodiment of the present invention. In this embodiment, the structure is substantially identical to that of FIG. 1 with the exception that the width of the primary guide 16 is tapered with decreasing width taken in the direction of power flow therein. In this manner, the field vector in the waveguide 16 is maintained constant as the power diminishes taken in the direction of power flow due to the siphoning of power into the resonator 2 via the coupling holes 22. Thus, a constant size coupling hole 22 will deliver uniform power to the resonator 2 over substantially the entire length ofthe resonator 2.

Referring now to FIG. 4, there is shown an alternative embodiment of the applicator of the present invention. The applicator 27 of FIG. 4 is essentially the same as that of FIG. 1 with the exception that the elongated resonator 2 is disposed over the broad wall 19 of the waveguide 16 such that the broad wall 5 of the resonator 2 is common to the top wall 19 of the waveguide 16. In addition, elongated ports are provided in the broad walls 19 and 21, of the waveguide 16 in alignment with the treatment zone such that the work piece 8 can be passed transversely through the treatment zone within the resonator 2 and through the feed waveguide 16. For this purpose, elongated port 7 in the broad wall 5 of the resonator 2 extends through the top wall 19 of the waveguide 16 and a third elongated port 28 is provided in the bottom wall of waveguide 16.

Microwave power is coupled from the waveguide 16 to the resonator 2 via an array of coupling holes 29 communicating through the adjacent broad walls of the resonator 2 and waveguide 16, such coupling holes 29 being spaced longitudinally along the guide I6 by approximately one-half a guide wavelength at the operating frequency of the applicator 27. The coupling holes 29 alternate from one side of the centerline of the guide 16 to the other side of the center line such that the coupling holes 29 couple energy from the primary guide 16 into the resonator 2 with the same phase throughout the length of the guide 16. Uniform power transfer through the coupling holes 29, as the power decreases taken in the direction of power flow through the feed guide 16, is accomplished by either increasing the size of the coupling holes 29 taken in the direction of power flow or by moving the holes 29 closer to the narrow walls 14 and 15 of resonator 2 where the magnetic fields are stronger. As in the applicator of FIG. I, the resonator 2 is excited in the TEM mode taken in the direction normal to the narrow walls 14 and I5, and the width of the resonator 2 is dimensioned to be cut off for the dominant waveguide mode therein.

Referring now to FIG. 5, there is shown an alternative embodiment to the applicator of FIG. I wherein a second feed waveguide 31 is positioned adjacent to the other narrow sidewall 15 of the resonator 2. As in the case of the applicator of FIG. I waveguide 31 is fed with power which is coupled through an array of coupling openings 32 in the narrow wall 15 of the resonator 2. The coupling openings 32 preferably being disposed at longitudinal positions intermediate the longitudinal positions of the coupling holes 22 in the opposed narrow wall 14 in order to obtain a more uniform distribution of power to the resonator 2.

Referring now to FIG. 6 there is shown an alternative embodiment to the applicator of FIG. 4 wherein a second feed waveguide 34 is positioned over the top broad wall 4 of the resonator 2, such second waveguide 34 being substantially the same as the first feed waveguide 16 and having an array of coupling holes 35 communicating between the second feed waveguide 34 and the cavity 2 for distributing microwave power from the second feed waveguide 34 to the cavity 2 to obtain a more uniform distribution of power thereto. As in the case of FIG. 5, the coupling holes 35 of the second array are spaced intermediate the position of the coupling holes 29 of the first array taken in the longitudinal direction of the waveguides I6 and 34.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

I. In a microwave applicator for treating material with microwave energy, an elongated hollow resonator for applying standing wave microwave electric fields to a relatively thin elongated treatment zone which extends longitudinally of said elongated resonator, an elongated waveguide means extending longitudinally of said elongated resonator for conveying microwave power therealong, coupling means distributed along the length of said resonator for providing distributed wave energy communication between said waveguide and said resonator for distributing microwave power from said waveguide to said resonator along the length of said resonator, the improvement wherein, said resonator means is dimensioned to be cut off for the dominant longitudinal waveguide mode therein at the operating frequency of the applicator, and wherein said coupling means is arranged for exciting a resonant TEM mode in said resonator to the exclusion of exciting other possible modes of oscillation, and wherein the reference direction for the TEM mode is a direction normal to the longitudinal axis of said resonator, whereby improved mode control is obtained.

2. The apparatus of claim I wherein said resonator includes a pair of ports disposed in opposite sidewalls of said hollow resonator in alignment with the treatment zone for passage therethrough of material to be treated.

3. The apparatus of claim 2 wherein said hollow resonator means is of generally rectangular transverse cross section having a pair of broad walls and a pair of narrow sidewalls, said pair of ports being disposed in said broad walls and being elongated in the longitudinal direction of said cavity, and the width of said broad walls is equal to or less than one-half the free space wavelength at the operating frequency.

4. The apparatus of claim I including a pair of wave energy absorbing means disposed within said elongated resonator at opposite ends thereof to obtain a nearly uniform microwave field near the ends of said resonator.

5. The apparatus of claim 1 wherein said waveguide means and said resonator means share a common wall, and wherein said coupling means includes an array of coupling holes communicating through said common wall.

6. The apparatus of claim 5 wherein said waveguide and 'said resonator are of rectangular transverse section each having broad walls interconnected by narrow walls, and wherein the width of said broad walls of said resonator is less than the width of said broad walls of said resonator is less than the width of said broad walls of said waveguide. I

7. The apparatus of claim 6 wherein said waveguide is tapered with decreasing width taken in the direction of power flow therealong to said resonator.

8. The apparatus of claim 5 wherein said common wall is a broad wall of said resonator and said waveguide.

9. The apparatus of claim 5 wherein said common wall is a narrow wall of said resonator and said waveguide.

10. The apparatus of claim 1 in which said resonator has two pairs of opposed elongated walls and the width of each wall of one of said pairs is equal to or less than one-half the free space wavelength at the operating frequency.

11. A microwave applicator for treating material with microwave energy comprising, a waveguide, wall means forming a cavity adjacent one side of said waveguide, means for coupling wave energy from said waveguide to said cavity, port means in said waveguide and said cavity walls through which material to be treated can pass through both said waveguide and said cavity.

12. A microwave applicator as claimed in claim 11 in which said waveguide and cavity have a common wall, said port means includes an elongated slot in said common wall, said coupling means comprises coupling holes in said common wall spaced along opposite sides of said slot with the coupling holes on one side of the slot being located between the coupling holes on the other side of the slot. 

1. In a microwave applicator for treating material with microwave energy, an elongated hollow resonator for applying standing wave microwave electric fields to a relatively thin elongated treatment zone which extends longitudinally of said elongated resonator, an elongated waveguide means extending Longitudinally of said elongated resonator for conveying microwave power therealong, coupling means distributed along the length of said resonator for providing distributed wave energy communication between said waveguide and said resonator for distributing microwave power from said waveguide to said resonator along the length of said resonator, the improvement wherein, said resonator means is dimensioned to be cut off for the dominant longitudinal waveguide mode therein at the operating frequency of the applicator, and wherein said coupling means is arranged for exciting a resonant TEM mode in said resonator to the exclusion of exciting other possible modes of oscillation, and wherein the reference direction for the TEM mode is a direction normal to the longitudinal axis of said resonator, whereby improved mode control is obtained.
 2. The apparatus of claim 1 wherein said resonator includes a pair of ports disposed in opposite sidewalls of said hollow resonator in alignment with the treatment zone for passage therethrough of material to be treated.
 3. The apparatus of claim 2 wherein said hollow resonator means is of generally rectangular transverse cross section having a pair of broad walls and a pair of narrow sidewalls, said pair of ports being disposed in said broad walls and being elongated in the longitudinal direction of said cavity, and the width of said broad walls is equal to or less than one-half the free space wavelength at the operating frequency.
 4. The apparatus of claim 1 including a pair of wave energy absorbing means disposed within said elongated resonator at opposite ends thereof to obtain a nearly uniform microwave field near the ends of said resonator.
 5. The apparatus of claim 1 wherein said waveguide means and said resonator means share a common wall, and wherein said coupling means includes an array of coupling holes communicating through said common wall.
 6. The apparatus of claim 5 wherein said waveguide and said resonator are of rectangular transverse section each having broad walls interconnected by narrow walls, and wherein the width of said broad walls of said resonator is less than the width of said broad walls of said waveguide.
 7. The apparatus of claim 6 wherein said waveguide is tapered with decreasing width taken in the direction of power flow therealong to said resonator.
 8. The apparatus of claim 5 wherein said common wall is a broad wall of said resonator and said waveguide.
 9. The apparatus of claim 5 wherein said common wall is a narrow wall of said resonator and said waveguide.
 10. The apparatus of claim 1 in which said resonator has two pairs of opposed elongated walls and the width of each wall of one of said pairs is equal to or less than one-half the free space wavelength at the operating frequency.
 11. A microwave applicator for treating material with microwave energy comprising, a waveguide, wall means forming a cavity adjacent one side of said waveguide, means for coupling wave energy from said waveguide to said cavity, port means in said waveguide and said cavity walls through which material to be treated can pass through both said waveguide and said cavity.
 12. A microwave applicator as claimed in claim 11 in which said waveguide and cavity have a common wall, said port means includes an elongated slot in said common wall, said coupling means comprises coupling holes in said common wall spaced along opposite sides of said slot with the coupling holes on one side of the slot being located between the coupling holes on the other side of the slot. 